1. Field of the Invention
The present invention relates to a package structure of a surface-mountable electronic component.
2. Description of the Related Art
Piezoelectric sound components have been widely used as a piezoelectric sounders for generating audible alarm or operational sound, a piezoelectric receiver, or other components in electronic apparatuses, home appliances, cellular phones, and so forth. A typical piezoelectric sound component includes a diaphragm fixed in a case closed with a cover, as disclosed in Japanese Unexamined Patent Application Publication No. 2000-310990.
While electronic apparatuses are being greatly reduced in size nowadays, the piezoelectric sound component is desired to be of a surface-mountable type capable of being directly mounted on a circuit board. For a surface-mountable electronic component, the case and the cover of the electronic component must have a heat resistance higher than a reflow temperature. Accordingly, the case and the cover are formed of a heat-resistant resin, such as LCP (liquid crystal polymer), SPS (syndiotactic polystyrene), PPS (polyphenylene sulfide), or epoxy resin.
Reflow temperature depends on the constituents of the solder used. Recent restriction of lead-containing solder has been expanding the range of application of lead-free solder. While lead-containing solder is reflowed in the range of 220° C. to 240° C., lead-free solder is reflowed at a temperature of 250° C. or more and requires high heat resistance of the package of electronic components accordingly.
In general, the heat resistance or softening temperature of a resin case and cover is set to be higher than a reflow temperature. The shapes of the case and cover are therefore maintained without being softened by reflow. Reflow using lead-containing solder does not seriously affect the package structure having the case and cover. However, if a case and a cover formed of a resin having a softening temperature higher than the reflow temperature are subjected to a reflow process at a high temperature, a stress resulting from the difference in thermal expansion between the case and the cover is placed on their bonded surfaces. The stress is likely to undesirably cause the case and the cover to separate during reflow or in a subsequent drop impact test. Thermal expansion of resins, particularly LCP, is varied depending on molding conditions, shapes, flow directions, and so forth even in the same material, and difference in thermal expansion among portions is consequently large. Therefore, the foregoing problems become more pronounced.